The fate of human skin grafts has been shown to be governed in man mainly by the HL-A chromosomal region. This includes many genes, some of which govern lymphocyte activation (LD genes) and others (SD genes) the antigens against which immunization occurs. A correlation between the length of skin or kidney graft survival and the number of SD incompatibilities has been found. The role of LD incompatibilities has not been established. Skin grafts in the four following situations will be performed: LD and SD different; LD different, SD identical; LD identical, SD different; LD and SD identical. To reach this end, LD typing will be done using LD homozygous cells in MLC. The other LD typing techniques, especially serological methods, will be investigated.